1. Field of the Invention
The invention relates to a continuously rotatable drum varyingly identified herein as an advance or forward or auxiliary gripper drum in sheet-fed rotary printing machines having a gripper bridge movable relatively to the drum.
2. Description of the Prior Art
An advance or forward gripper driven by means of a four-bar linkage chain and without a cam drive has become known heretofore from the German Petty Patent DT-Gbm 7 131 281. This petty patent is concerned with a reciprocatingly rocking advance or auxiliary gripper which requires very long periods of time from the instant the sheet is gripped to the instant it is transfered. Since, on the other hand, the time period between the instant of sheet transfer to the impression cylinder and the instant the gripper rocks back to its starting position is kept much too short, a very long cylinder channel, extending over almost 200.degree., must be provided in this heretofore known device of the aforementioned German petty patent whereby, in turn, the paper support surface and, therewith, the format length to be processed are considerably shortened. Moreover, poor transmission or transfer relationships are provided due to a relatively small transmission or transfer angle in this known advance or forward gripper device of the aforementioned German petty patent.
In German Petty Patent DT-Gbm 7 217 700 and in German Published Prosecuted Application DT-AS 1 118 811, gripper devices for continuously rotating transfer or sheet feeding cylinders are described wherein the gripper bridge is pivoted or rocked in such manner that the gripper support has zero absolute velocity at the sheet take-up. The pivoting or rocking movement of the gripper bridge in the last-mentioned German petty patent and in the German published application is effected by means of a cam gear and, in the case of the German published application, in conjunction with fixed bolts and pins extending into the fork mouths of control forks.
At high printing speeds, the devices of the foregoing last-mentioned German petty patent and the German published application are subject to limitations with respect to the accuracy of registry during sheet take-up and transfer. Due to the required high angular velocities and angular accelerations of the gripper bridge, large inertial forces are generated, whereby the cam members and cam followers are necessarily subjected to high loads. In order to prevent the cam follower from lifting off the cam member and to ensure force-locking or positive connection therebetween during the required deceleration of the gripper device, powerful springs must be provided, however, such springs constitute an additional load in the acceleration phases of these cam gears.